Historically, communications switches were designed to manage and perform most, if not all, the functions associated with the completion of a telephone call. These functions include switching, routing, signaling, call control and service control. The development of digital, software-driven switches, allowed the signaling function to be decoupled from the other functions performed by a switch. Accordingly, protocols governing the format and the type of signaling information, were developed to a) allow communications between switches and b) manage the interface between a switch and a signaling node handling some of the signaling functions. Thus, these interfaces were limited to strictly network-specific functions and accordingly they became known in the art as "Network Node Interfaces" (NNI). The set of standardized protocols associated with the NNI type of interfaces is the Signaling System No. 7 (SS7).
As customer premises equipment, such as Private Branch Exchanges (PBX) and telephone sets, started to use digital signals, as opposed to analog signals, to communicate with switches, protocols were also needed to manage the communications interfaces between users' equipments and switches. Accordingly, protocols were also developed to standardize user-network access procedures or, more specifically, the interfaces between the customer premises equipment and the switches of a telephone network. Those interfaces became to be known in the art as "User-Network Interfaces" (UNI). Associated with the UNI interfaces is a standardized set of protocols known as "Digital Subscriber Signaling System No. 1" (DSS1).
With the introduction of Narrowband Integrated Services Digital Network (N-ISDN) and the Open Systems Interconnection (OSI) model which defined user-network access and network-to-network (or switch-to-switch) standards for voice and data communications, UNI and NNI were adapted to conform to these new standards. NNI protocols were further extended to standardize communications procedures between switches and service-specific database systems that provide, for example, call related information look-up functions for services, such as Freephone, known in the United States as 800 Service. The subset of the NNI protocol suite related to communications between switching nodes and service-specific processors in a network is known as Transactions Capability Application Part or TCAP for short. Unfortunately, the dichotomy between user-network access procedures and switch-to-switch communications procedures remained. A consequence of that dichotomy is manifested in the need for multiple protocol conversions to take place (from UNI format to NNI format and back to UNI format) in order to relay, for example, caller-id information to the called party. More importantly, ISUP messages still include information and related parameters associated with different call processing functions, thereby unduly complicating execution of call processing functions.
With the development of Broadband Integrated Services Digital Network (B-ISDN) standards and their anticipated widespread implementation to offer a wide variety of services using, for example, Asynchronous Transfer Mode (ATM) switches, it is apparent that the existing signaling protocols are inadequate to support those new services. Extension of the existing ISDN signaling protocols to support the B-ISDN services presents certain drawbacks. Those drawbacks include a) perpetuating the distinction between user-network interfaces and network-to-node interfaces, which is likely to be dysfunctional in an environment where the roles of, and the relationship between, switches, customer premises equipment and other processors in a network can be redefined for a particular service; b) perpetuating the bundling of certain functions, such as call control, connection control, and channel control in the NNI and UNI protocol suites; and c) difficulty in managing connections, in general, and meeting quality-of-service parameter checking requirements for certain connections, in particular.
Consideration has also been given to derive new message syntax and message flow procedures for B-ISDN networks. However, that approach precludes the reuse of existing protocol-related software. Thus, no prior art signaling protocol architecture is able to provide a feasible, modular and flexible approach to allow widespread implementation of the emerging B-ISDN services while accommodating traditional and N-ISDN telephony services.